It is well known to construct vehicle body door, hoods, and deck lids by stamping an outer panel and an inner panel which are joined together by flanging the edges of the outer panel over the edge of the inner panel. The inner and outer panels are individually stamped to their desired size and shape, with the outer panel being slightly larger than the inner panel to provide an edge portion along the edge of the outer panel which can be folded over the edge of the inner panel to define the hem flange which connects the two panels.
It has been recognized in the prior art that this hem flanging together of the panels may not be sufficient to prevent the inner panel from sliding relative to the outer panel. Accordingly, it has been known to employ auxiliary attachment techniques to lock the panels against relative movement For example, it has been known to apply an arc, mig, or fusion weld to tack the hemmed over edge of the outer panel to the inner panel. It is also known to spot weld the two panels together at the flange. In still other situations, induction heat has been used to cure the adhesive previously applied between the panels at the area of the flange. In each of these cases, these operations requires additional equipment and labor and inevitably damages or distorts the assembled panels which in turn require refinishing of the metal to correct the imperfections which will become visible when the outer panel is painted.
It has also been known to provide a hole in the edge of the inner panel and to provide a corresponding projection on the face of the hem flanging punch so that the punch which flanges the outer panel over the inner panel will also press the flange into the hole in the inner panel. Although this technique eliminates the additional investment and labor of a welding operation, it nonetheless has been found to distort the outer panel because the hole in the inner panel reads through the outer panel.
It would therefore be desirable to provide an improved method for interlocking together the inner and outer panels of a hemmed flange motor vehicle closure panel.
According to the present invention, the method of interlocking the hemmed together flanges of the inner and outer panels comprises the steps of forming a plurality of raised beads at spaced intervals along the length of the edge of the inner panel; placing the inner panel inside the outer panel with the raised beads facing away from the outer panel; and hemming the edge of the outer panel over the edge of the inner panel by a hemming punch having a plurality of slots therein located to register with the raised beads of the inner panel so that the outer panel is coined into interlocking engagement with the raised beads to thereby interlock the inner and outer panels together against relative movement. The raised beads preferably have a height at least equal to the thickness of the inner panel, and are preferably elongated in shape with the elongation extending in the direction either parallel or perpendicular to the edge of the panel. The slots in the hemming die preferably have a length longer than the mating raised bead of the inner panel to allow at least one millimeter of clearance between the panels at each end of the beads.